The present invention relates to an electrophotographic liquid developer containing negatively charged toner particles and comprising a carrier liquid having a high electrical resistivity and a low dielectric constant, a pigment or dye constituent, a resinous binder, a charge controller and conventional additives. The invention also relates to a process for preparation of such a liquid developer.
In the field of electrophotography, processes which have found widespread application are generally those in which the toner particles deposited on the electrostatic latent charge image are transferred from the photoconductor layer to a suitable transfer material, such as plain paper, and are fixed thereon. In such procedures, the charge image can be developed using either dry or liquid developers.
Basically, liquid developers comprise insulating carrier liquids in which pigments or dyes, resins, charge controllers and possibly other additives are dispersed or dissolved. In the electric field of the charge image, the charged toner particles are deposited electrophoretically on the charge image. For positive charge images, such as those formed in electrophotographic processes using photoconductive selenium layers, developers containing negatively charged toner particles are required. In order for the transfer from the copying layers to the transfer material to be successful, the toner particles must be larger than non-transferrable toner particles, utilized for developing charge images on zinc oxide/ binder layers.
Various liquid developers with negatively charged toner particles are known for use in the toner image transfer process. British Pat. No. 1,374,701 discloses a process in which a pigment is milled or kneaded with a combination of three polymers A, B and C in an aromatic solvent, and the ground material is then dispersed in an insulating carrier liquid in which the polymers are insoluble or only slightly soluble. Polymer A effects good transfer of the toner particles; polymer B ensures high stability of the liquid developer and good fixability on the transfer material, and polymer C controls the electrical charge of the toner particles.
U.S. Pat. No. 3,853,554 discloses liquid developers containing toner particles comprising pigments/dyes, cyclized rubber, polyethylene and a charge controller--in particular, for a negative charge, lecithin--and further resins and waxes. To prepare the liquid developer, the pigments/dyes are initially dispersed in a solution of cyclized rubber and of the further constituents in an aromatic solvent. The dispersion is then diluted with the insulating carrier liquid which does not dissolve the rubber.
U.S. Pat. No. 3,993,483 discloses a liquid developer which is prepared by dispersing pigment in an aromatic solution of two polymeric compounds, after which the dispersion is diluted with the nondissolving insulating carrier liquid. One polymer is a varnish resin, for example a coumarone, phenol or maleate resin, and the other is a polyolefin or an olefin copolymer. To obtain negatively charged toner particles, various controllers, such as carrier liquid-soluble metal salts of organic sulfoacids, are added.
It is also known to prepare liquid developers by distributing the pigments in dispersions of polymers in the insulating aliphatic carrier liquid (so-called organosols or dispersimers) and thereafter effecting dilution. Pigments flushed with resin solutions are preferably used. During the process in which the pigment surface is treated, further polymerization of monomer onto the resin used can be effected. The dispersimers are obtained by a multistage copolymerization and graft polymerization (U.S. Pat. No. 4,081,391) or by dissolution of a resin, which is insoluble in the carrier liquid, in a monomer or mixture of monomers which is capable of forming a soluble polymer, and subsequent polymerization (U.S. Pat. No. 4,104,183). The dispersimers preferably also contain waxes such as polyethylene waxes. Liquid toners of this type can also contain two dispersimers (German Offenlegungsschrift No. 2,936,042). The toner charge is determined by the composition of the dispersimers and of the pigments.
U.S. Pat. No. 4,161,453 discloses preparation of liquid developers by kneading the pigment with a molten copolymer of styrene and allyl alcohol (or an allyl alcohol ester), then grinding the kneaded material, after cooling, to give a dry toner, and finally dispersing the dry toner particles in a carrier liquid which contains the charge controller and soluble acrylic resins.
U.S. Pat. No. 4,243,736 and German patent application No. P 30 11 193 disclose further liquid developers for the toner image transfer process which essentially contain the carrier liquid, pigments and N-vinylpyrrolidone-containing copolymers which are soluble in the carrier liquid and which simultaneously act as the dispersant or fixer and as the charge controller. The polymers can be copolymers, terpolymers or graft copolymers.
Although in many cases the known liquid developers which are suitable for the toner image transfer process give good copies and large numbers of copies, they have various deficiencies. Some of the liquid developers contain physiologically unacceptable aromatic solvents. During the copying process, these solvents pass, together with the carrier liquid, into the air surrounding the copying machine. In addition, the aromatic compounds also adversely affect the odor of the developers. Depending on the distribution of the aromatic solvents between the insulating carrier liquid and the polymer particles, they additionally increase to a greater or lesser extent the tackiness of the toner particles. This is disadvantageous for the stability of the liquid developer, since tacky particles collect together more readily to give undesirable agglomerates. Other known liquid developers have complicated compositions. In addition to the dispersimers which often must be obtained by a multi-stage reaction, they also frequently contain pigments which are pre-wetted by the flushing process. Because of the complicated and time-consuming nature of the preparation of the starting materials and of the production processes, these developers are relatively expensive. This also applies to liquid developers obtained by dispersing a dry toner in a carrier liquid. On the other hand, the known liquid developers which are of simpler composition containing negatively charged toner particles for the toner image transfer process exhibit deficiencies when used for a relatively long time in copying machines. They soil the machines to a relatively great extent, thus giving rise to breakdowns during the copying process and to a reduced quality of the copies.